This invention relates to an engine bearing alloy composed primarily of aluminum and lead which has a microstructure typified by fineness and uniformity of the lead phase previously unattainable in a product originating as cast strip. This invention also involves the method of making the alloy so as to obtain the desirable metallurgical microstructure. This method involves casting the alloy in strip form using a high quench rate process.
Most engine bearing alloys have a metallurgical microstructure consisting of a soft, low-melting-point phase uniformly distributed throughout a relatively hard and strong matrix. The soft phase consists of innumerable small islands of the order of 0.001-0.1 mm in size, which may or may not be interconnected. When the matrix is aluminum or aluminum strengthened by minor additions of alloying elements, the soft phase is commonly tin. The volume percentage of soft phase is 10-40% in the copper-lead system, and 2-20% in the aluminum-tin system. At the lower end of each range it is common practice to augment the bearing properties of the alloy by providing the bearing with a soft, thin overlay of a lead-base alloy.
Aluminum-lead is a desirable engine bearing alloy with good bearing properties. Moreover, lead is a less expensive metal than tin. The extent to which aluminum-lead had been adopted as an engine bearing alloy in practice, however, has been limited by the metallurgical problems associated with production of the alloy. In order to obtain a fine distribution of lead throughout an aluminum matrix it is first necessary to dissolve the lead in molten aluminum. The melt temperatures necessary to do this are much higher than are commonly used in aluminum casting practice. The higher the proportion of lead it is desired to incorporate, the higher must be the melt temperature. A greater difficulty arises during the cooling prior to solidification of the alloy. As the melt cools, lead is precipitated in the form of discrete droplets of molten lead. The specific gravity of these droplets is very much higher than that of the surrounding molten aluminum. Even though the molten aluminum still contains some lead in solution, its specific gravity is less than one quarter that of the molten lead droplets. The droplets, therefore, fall through the molten aluminum under the influence of gravity. When the aluminum freezes, a preponderance of lead is found at the bottom, while the top is denuded of lead. The desirable uniform distribution of lead has thus not been achieved.
Various methods of producing aluminum-lead bearing alloys have been proposed, some of which have been put into practice with varying degrees of success. These methods are briefly explained below.
One method (as shown in U.S. Pat. Nos. 3,410,331; 3,545,943; 3,562,884; 3,580,328; and 3,667,823) consists of dissolving lead in molten aluminum and horizontally continuous casting the alloy. The cast alloy is then rolled and roll-bonded to steel, and steel-backed bearings are formed from the resulting bimetal. Owing to the lead segregation phenomenon described above, the underside of the cast is lead-rich, the top side is denuded of lead and there is a lead gradient through the thickness of the cast. The low-lead side of the rolled alloy is used for bonding to the steel backing, the high lead side being partially removed during the machining of the bearing to its final dimensions. The amount of lead appearing in the machined bearing surface depends on the nature of the lead gradient in the as-cast alloy and on the thickness of the finished bearing lining relative to the as-bonded lining thickness. Achievement of a desirable known and constant lead content in the bearing surface requires very close control of the casting conditions such that the lead gradient has a constant value, and close control of the bonding conditions, such that the thickness reduction of the steel backing during bonding is known precisely. Such close control is in fact difficult to achieve in practice, and represents a significant drawback to the process. A further drawback is that the problem of controlling the lead gradient becomes more difficult as the percentage of lead incorporated in the melt is increased. Although U.S. Pat. No. 3,410,331 speaks of a process in which the lead content of the melt is up to 15% by weight (4% by volume),in practice the process has not been operated in production with lead contents above 6% by weight (1.5% by volume) because of the aforementioned difficulty. Yet a further drawback to the process is that the lead spheres in the as-cast alloy may be larger than desirable. A typical size range is 20-200 microns. During the rolling and roll-bonding processes the alloy is reduced in thickness by a factor of about ten to about twenty, and elongated by the same factor. The spherical lead particles may become elongated into ribbons 400-4000 microns in length. Lead in this form is considered undesirable for certain applications since it leads to a lowering of the fatigue strength of the bearing lining.
A second method is shown in U.S. Pat. No. 3,495,649 and consists of dissolving lead in molten aluminum and vertically continuously casting the alloy. Segregation of lead droplets occurs in this process also, the first alloy cast being lead-rich, and the last alloy cast being denuded of lead. In theory, equilibrium is achieved for the major part of the cast and, except for the beginning and the end, the alloy contains a uniform lead content. In practice, any change in the cooling conditions in the casting die results in a change in the rate of segregation and a variation in the lead content. For this and other metallurgical reasons, the process has not been put into production.
Another method (shown in U.S. Pat. No. 3,432,293) consists of dissolving lead in molten aluminum and solidifying while the melt is falling freely, like a waterfall, under the influence of gravity. Under such conditions there is no tendency for the lead to segregate and a uniform distribution of lead is in theory obtained. In practice, the problems of uniformly freezing a free-falling stream of molten aluminum are formidable, and the invention has not been realized in practice. Other proposals, such a solidifying the melt in space, away from the influence of the earth's gravitational field, have proved even more impracticable.
Yet another method (shown in U.S. Pat. No. 4,069,369) consists of dissolving lead in molten aluminum and atomizing a stream of the molten metal to powder. Each atomized particle freezes very quickly and the precipitated lead is distributed uniformly within each particle. The powder is then consolidated by rolling into a strip, which is sintered and roll-bonded to steel. The process produces an aluminum-lead bimetal lining without a lead gradient, but which still contains undesirable lead ribbons up to 500 microns in length. The process is, moreover, unattractive in that there are several process stages, making the overall process costs relatively high.
A final method consists of mixing aluminum and lead powders, together with other minor additions, in powder form, spreading the powder onto steel, roll compacting and sintering. There resultant bimetal strip lining has no lead gradient and contains no lead ribbons of significant length. The process economics are good. However, the fatigue strength of the alloy produced in this way is likely to be adversely affected because of the oxide coating on the individual aluminum-lead powder particles. Such bearings are considered suitable only for lightly loaded applications.